The objectives of this research proposal are to: 1) Determine the incidence of asymptomatic extracranial carotid occlusive disease in high risk patients undergoing major operations; 2) Determine if perioperative stroke in these patients is related to detectable antecedent advanced carotid obstruction, and 3) Determine if prophylactic carotid endarterectomy of high-grade carotid stenosis reduces the risk of perioperative stroke in patients undergoing major operation. This research will employ noninvasive techniques (plethysmography, doppler ultrasound, and ultrasonic arteriography) to detect asymptomatic significant greater than or equal to 50% stenosis or occlusion of the extracranial internal carotid artery in the following three groups of patients: 1) All patients undergoing coronary artery bypass grafts; 2) All patients undergoing major reconstruction for peripheral arterial occlusive diseases; and 3) All patients with symptomatic carotid bruits who are candidates for major general surgical procedures. Patients with noninvasive evidence of advanced carotid disease will undergo contrast arteriography and those with significant stenoses will receive random, prophylactic endarterectomy or anti-platelet therapy prior to the indicated major operation. All patients will be followed for perioperative (thirty-day) and late (up to five years) death or stroke (major end points) and transient ischemic attacks ( minor response variable).